This disclosure concerns an optical interference filter wherein rotationally displaced birefringent retarders and associated polarizers discriminate light by wavelength, with a comb filter transmission characteristic.
The disclosed filter has a minimum number of birefringent retarder elements, in particular only from two to four, which can have liquid crystals for tuning. Empirical solutions for rotational orientation of the retarders are disclosed that are not integer fractions of π radians. The filters nevertheless have good in-band versus out-of-band discrimination, and are tolerant of rotational alignment errors. The filter is useful for operation in infrared wavelength bands. The minimum number of interfaces reduce the incidence of reflection and absorption. Materials are disclosed that optimize the filter for these bands.